For example, in servers, electronic elements such as CPUs are accompanied by extremely large heat generation as the processing capability of the server is improved (high-speed processing). Therefore, a cooling device for cooling the electronic element is indispensable for securing the operation stability of the whole system.
For example, a cooling device for cooling an electronic device such as a server has been configured as follows until now.
In other words, the cooling device includes a heat receiver, a heat radiator connected to a discharge port of the heat receiver via a heat radiation path, and a return path connecting the heat radiator and an inflow port of the heat receiver with each other. The heat receiver includes a heat receiving plate which is in contact with the heat generation body to absorb heat and a heat receiving cover which covers the surface of the heat receiving plate and forms a heat receiving space for evaporating the refrigerant flowing on the surface. In addition, in the heat receiving space, the heat receiving plate includes a refrigerant inflow portion near a center thereof and a vaporizer in which radial groove portions are provided toward an outer periphery of the refrigerant inflow portion, and an introduction pipe is configured to be disposed in a direction substantially perpendicular to the heat receiving plate (see, for example, Japanese Patent Unexamined Publication No. 2009-88127).
In the cooling device of Japanese Patent Unexamined Publication No. 2009-88127, there is a case where the cooling performance is reduced in a case where a heat generation amount of the electronic element that generates heat by being in contact with the heat receiver is small.
In other words, in the cooling device of the example of the related art, a portion of the refrigerant flowing into the refrigerant inflow portion from the introduction pipe receives heat from the heat receiving plate and boils and vaporizes. At this time, the boiled and vaporized refrigerant diffuses as a high-speed mixed phase flow (gas phase and liquid-phase) on the heat receiving plate together with the unboiled liquid-phase refrigerant due to rapid volume expansion. At this time, the unboiled liquid-phase refrigerant also spreads in the form of a thin film on the entire surface of the heat receiving plate. Then, by continuous heating from a heat generation body, the unboiled liquid-phase refrigerant is instantaneously heated and vaporized, thereby efficiently removing vaporization heat from the entire heat receiving plate and cooling the heat receiving plate.
However, although the cooling device of the related art shows very good performance in a case where the heat receiver receives a large heat amount from the electronic element, in a case where the heat amount is small, as described above, an evaporation amount of the initial boiling of the refrigerant generated on the heat receiving plate near the tip of the introduction pipe in the heat receiver is also reduced, and thus the cooling performance is reduced. This is due to the fact that the refrigerant cannot obtain a sufficient volume expansion rate because the evaporation amount of the refrigerant is reduced. In other words, since the speed of volume expansion of the refrigerant is slow, a film thickness of the liquid-phase refrigerant formed on the heat receiving plate cannot be reduced, resulting in a decrease in cooling performance. This reduction in the evaporation amount of the refrigerant is due to a significant decrease in a bubbles generation amount from the refrigerant due to the low heat generation amount of the electronic element. Therefore, in a case where the heat generation amount of the electronic element is large, although the superiority of the cooling device of other type (air cooling, water cooling, or the like) is large, in a case where the heat generation amount of the electronic element is small, the temperature rise of the heat receiver relatively decreases and the superiority over the cooling method of another type is greatly diminished.
Therefore, it is proposed that a hydrophilic surface treatment film is formed on the surface of the heat receiving plate by a laser processing method described in Shohei Umemoto et al. “Passive two-phase cooling using super-hydrophilic boiling surface and refrigerants for electronic devices”, Proceedings of the 53rd Transmission Symposium (2016-5), C 134as one means of generating a lot of bubbles from the refrigerant even in a case where the heat generation amount of the electronic element is small. By forming a hydrophilic surface treatment film on the surface of the heat receiving plate, even in a case where the heat generation amount of the electronic element is small, generation of bubbles from the refrigerant is promoted, and as a result, since the evaporation amount of the refrigerant is increased, it is reported that efficient cooling is possible.
However, the hydrophilic surface treatment film formed on the surface of this heat receiving plate is a very thin film. In addition, there is a case where fine foreign matters may be mixed in the refrigerant or the device. Therefore, there is a high possibility that the surface treatment film is physically damaged by fine foreign matters flowing together with a high-speed refrigerant flow generated on the film surface when the refrigerant is boiled. Therefore, in order to achieve improvement of both cooling performance of the cooling device and long-term operation stability, securing long-term durability of the film has been a problem.